1. Field of the Invention
This invention relates to an objective lens driving device for condensing a laser light radiated from a light source, such as a semiconductor laser, onto a signal recording surface of a disc-shaped optical recording medium, such as an optical disc or a magneto-optical disc, or controlling an objective lens to cause the laser light to follow a recording track of the optical disc, and a method for producing a resilient supporting member employed in such objective lens driving device for elastically movably supporting a lens holder.
2. Description of the Related Art
Heretofore, an optical disc player, an optical reproducing apparatus or an optical recording/reproducing apparatus includes an optical pickup device for radiating a laser light on the signal recording surface of an optical disc as a recording medium for reading out information signals recorded on the optical disc or recording information signals on the signal recording surface of the optical disc as well as detecting the return light from the signal recording surface for reading out or reproducing the information signals.
Such optical pickup device includes a light source for radiating the laser light to be illuminated on the signal recording surface of the optical disc, an objective lens for condensing the laser light radiated from the laser light source on the signal recording surface of the disc, and light detectors, such as photodetectors, for detecting the return light reflected back from the signal recording surface of the optical disc and traversing the objective lens.
The optical pickup device includes an objective lens driving device for driving the objective lens highly accurately for recording information signals on a recording track of an optical disc or reading out information signals recorded with high density on the signal recording surface of the optical disc. With such objective lens driving device, the objective lens, on which the laser light is incident, is controlled highly accurately for condensing the laser light radiated from the laser light source and for causing the laser light to precisely follow the recording track on the optical disc.
That is, with the present objective lens driving device, the objective lens for condensing and illuminating the laser light on the signal recording surface of the optical disc is driven and displaced in two mutually perpendicular directions, that is in a focusing direction parallel to the optical axis of the objective lens and in a tracking direction normal to the focusing direction, on the basis of the focusing control signal and the tracking control signal. This causes the laser light to be correctly condensed on the signal recording surface of the optical disc and to correctly follow the recording track of the optical disc.
As an example of the conventional objective lens driving device, there has been known a wire-supported objective lens driving device 100 in which a tubular lens holder 102 mounting an objective lens 101 is supported in a cantilevered fashion by upper and lower and left and right paired resilient supporting members 110A to 110D, as shown in FIG. 1. Each of the four resilient supporting members 11A to 110D has its free end secured to the lens holder 102, while having its proximal end secured to a mounting plate 104 set upright on a supporting substrate 103. Consequently, the objective lens 101 is adapted for being resiliently displaced by the resilient displacement of the resilient supporting members 110A to 110D in two axial directions, namely in the direction along the optical axis and in the direction normal thereto, in unison with the lens holder 102.
The above-described conventional wire-supported type objective lens driving device 100 has an electro-magnetic driving device for driving the lens holder 102 in the focusing direction parallel to the optical axis of the objective lens 101 and in the tracking direction normal to the optical axis. The electro-magnetic driving device includes a focusing driving coil 105 placed on the outer peripheral surface of the lens holder 102, a tracking driving coil 106 joined on the focusing driving coil 105 and a magnetic circuit made up of a pair of magnets 107A, 107B arranged on the supporting substrate 103 and a pair of yokes 108A and 108B arranged facing these magnets 107A and 107B. The magnets 107A, 107B and the yokes 108A and 108B, making up the magnetic circuit, are arranged on the supporting substrate 103 for facing the focusing driving coil 105 and the tracking driving coil 106. On the mounting plate 104 is assembled a circuit substrate 109.
The above-described objective lens driving device 100 is assembled on an optical pickup unit which is made up of a laser light source and an optical system or a light sensor, with the laser light being emitted by the laser light source so as to fall on the objective lens 101. The laser light falls on the optical disc via the objective lens 101 and is thereby reflected by the signal recording surface of the disc so as to be again incident on the objective lens 101 as the return light. The optical pickup device detects by the light sensor the focusing error signal and the tracking error signal along with the information signals from the return light.
The objective lens driving device 100 is fed by a controller with the control current corresponding to the focusing error signal or the tracking error signal for driving the focusing driving coil 105 or the tracking driving coil 106. The resilient supporting members 110A to 110D are resiliently displaced in the focusing direction or in the tracking direction by such driving of the focusing driving coil 105 or the tracking driving coil 106 for displacing the objective lens mounted on the lens holder 102 in two directions, namely in the direction along the optical axis and in a direction normal thereto. By such displacement of the objective lens 101, the optical pickup unit performs focusing control and tracking control of the laser light with respect to the optical disc.
With the above-described conventional objective lens driving device 100, it may occur that, when the laser light is caused to follow the recording track of the optical disc by the focusing control and the tracking control, the lens holder 102 on which the objective lens 101 is assembled tends to be deviated significantly due to severe vibrations of the resilient supporting members 110A to 110D. It is occasionally not possible with the objective lens driving device 100 to drive the objective lens 101 correctly in the focusing direction and in the tracking direction with respect to the optical disc, such that the information signals cannot be recorded or reproduced correctly.
That is, with the resilient supporting members 110A to 110D provided on the conventional objective lens driving device 100, metal wires are generally employed which are circular in cross-section and whose spring constants in the focusing direction and in the tracking direction are equal to each other. These resilient supporting members 110A to 110D have natural first-order resonant frequencies, as determined by characteristics proper to the spring material, which are substantially equal in the focusing direction and in the tracking direction. If, with the objective lens driving device 100, the above-described focusing operation or the tracking operation is performed at a frequency comprised within the natural first-order resonant frequency proper to the resilient supporting members 110A to 110D, the resilient supporting members are vibrated in resonance at larger amplitudes thus rendering it impossible to stably control the objective lens.
The conventional practice for combatting such problem has been to connect a buffer member to the resilient supporting members 110A to 110D for shifting the first-order resonant frequency of the resilient supporting members 110A to 110D to outside the frequency range of the focusing control operation or the tracking control operation.
However, these measures lead to increased production cost of the objective lens driving device 100. In addition, the above effect cannot be achieved unless the buffer material is securely and intimately fastened to the resilient supporting members 110A to 110D. The result is that the buffer material joining operation necessitates an extremely laborious operation by a skilled operator, while it cannot be carried out on the objective lens driving device without considerable difficulties.